1. Field of the Invention
The present invention relates to a novel tolane derivative used as a component of electro-optic display materials, a liquid crystal composition containing the derivative and a liquid crystal display device using the composition.
2. Description of the Prior Art
Liquid crystal display devices utilize the electro-optical effect of the liquid crystal. The display modes currently used include the twisted nematic (hereinafter referred to as TN) mode and the super twisted nematic (hereinafter referred to as STN) mode which has a larger twist angle. The characteristics required for these display modes are as follows:
1. Colorless and thermal-, photo-, electrical- and chemical stabilities; PA0 2. Wide practical temperature range; PA0 3. Electro-optical high-speed response; PA0 4. Low driving voltage; PA0 5. Rapid rising of voltage-light transmittance characteristic, and small temperature dependency of the threshold voltage (hereinafter referred to as Vth); and PA0 6. Wide visual angle.
A number of liquid crystal compounds which satisfy characteristic 1 are well known. However, no single liquid crystal compound is known which satisfies characteristics 2-6. Therefore, in order to satisfy these characteristics, liquid crystal compositions in which several kinds of nematic liquid crystal compounds or nonliquid crystal compounds are blended have been used.
For example, in order to satisfy characteristic 2, a liquid crystal compound is required which has not only a low crystal phase-nematic liquid crystal phase transition point (hereinafter referred to as C-N point) but also a high nematic liquid crystal phase-isotropic liquid transition point (hereinafter referred to as N-I point) and consequently a wide temperature range of nematic liquid crystal phase.
On the other hand, in order to satisfy characteristic 3, i.e., to make the response speed fast, the cell gap should be decreased in the relation between the response speed (hereinafter referred to as .tau.), the viscosity coefficient (hereinafter referred to as n) and the cell gap (hereinafter referred to as d) represented by the following equation: EQU .tau..varies.nd.sup.2
In the cell practically used, in order to prevent the occurrence of interference fringes on the surface of the cell which causes damage to the cell appearance, the .DELTA.n.multidot.d is adjusted to a constant value. .DELTA.n means the anisotropy of refractive index; the same applies to the other .DELTA.n hereinafter. Therefore, by using a material having a high .DELTA.n value, the d value can be made small and, consequently, the response speed can be increased.
In order to satisfy characteristics 2 and 3 simultaneously, there are known tolane derivatives which are described in Adv. in Liquid Crystal Research and Application (edited by L. Bata), Oxford: Pergamon Press; Budapest: Akademiai Kiado 1980, S. 1029; Japanese Patent Application Laid-Open No. 305040/1989; and so on, as liquid crystal materials having not only a wide temperature range of nematic liquid crystal phase but also a large .DELTA.n value.
TABLE 1 __________________________________________________________________________ Transition No. Structural Formula Point Literature __________________________________________________________________________ (i) ##STR2## C-N point: 100.degree. C. N-I point: 94.degree. C. Adv. in Liquid Crystal Re- search and Application. (ii) ##STR3## C-N point: 77.degree. C. N-I point: 152.degree. C. Adv. in Liquid Crystal Re- search and Application. (iii) ##STR4## C-N point: 179.degree. C. N-I point: 46.degree. C. Japanese Patent Application Laid-open No. 305040/1989 (iv) ##STR5## C-N point: 152.degree. C. N-I point: 16.degree. C. Japanese Patent Application Laid-open No. 305040/1989 (v) ##STR6## C-N point: 151.degree. C. N-I point: 12.degree. C. Japanese Patent Application Laid-open No. 305040/1989 __________________________________________________________________________
It is described in the above literature that compounds (i) and (ii) have a nematic state in a wide temperature range and have no smectic state, and that compounds (iii), (iv) and (v) have well-balanced desirable properties as liquid crystal components such as an extremely large .DELTA.n value, a low viscosity for compounds of three-ring structure and a high N-I point, respectively.
However, in compounds (i) and (ii), the .DELTA.n is expected to become smaller, since they have a chloride group or nitrile group in the side chains of their skeletons. On the other hand, compounds (iii), (iv) and (v) have C-N points of 150.degree. C. or higher, and consequently it appears that the compounds have poor compatability with other liquid crystal compounds.